Enzymes are proteins which catalyze chemical reactions. Their actions generally are quite specific. For example, glucoamylase is an enzyme which catalyzes the saccharification of a starch hydrolyzate, the enzyme substrate, to glucose (dextrose), the end product. Glucose is widely used in the manufacture of fructose from starch.
Fructose is an isomer of dextrose that is sweeter than dextrose, and, therefore, it is widely used as a sweetener. In the manufacture of fructose, starch, e.g., corn starch, is hydrolyzed by acid or enzyme thinning to a dextrose equivalent (D.E.) of, e.g., 10, and then subject to enzymatic saccharification with glucoamylase to produce a product having a D.E. above 95, preferably as high as possible. The product, which can be called dextrose, is isomerized with glucose isomerase to produce fructose.
In conventional technology, glucoamylase is used on a batch basis to react with starch hydrolysis products. At the end of the reaction, as in many enzymatic reactions, it is difficult to recover the glucoamylase, and therefore a fresh supply must be used for each batch. As in the case of many enzymatic reaction, it has been recognized that this expense can be avoided if the glucoamylase can be immobilized on a support that is contacted with the starch hydrolyzate and then recovered. However, attempts to immobilize glycoamylase on porous supports in the past have failed to yield an enzyme capable of converting available starch to sugar at rates comparable to the free enzyme.
In order for an enzyme to be immobilized on a support and function in a satisfactory manner, certain requirements must be satisfied. First, the efficiency of the enzymatic reaction should not be diminished to any significant degree. Second, the supported enzyme must be durable, so that it will preserve its activity through several reactions. Third, the cost of immobilizing the enzyme must be reasonable, so that the overall cost of conducting the enzymatic reaction is not increased, and preferably is significantly reduced.
Many attempts to immobilize enzymes on solid supports have involved the use of porous supports. Porous supports are selected because they have large surface areas in proportion to the volume of the support (the surface area:volume ratio is high). For example, controlled pore glass has been described extensively in the literature as a support for enzymes. However, the diameters of the pores in such supports generally are small, and therefore it may be difficult for large molecules such as starch hydrolyzate to penetrate them. On the other hand, non-porous supports, which do not have this difficulty, have relatively low surface area:volume ratios, and therefore enzymes immobilized on them have relatively low activity.
Controlled pore glass also is quite expensive. Therefore, the potential cost advantages of immobilization of enzymes may not be achieved when it is used, especially with relatively inexpensive enzymes. While less expensive porous supports such as alumina also have been described in the literature, they nevertheless suffer from the other disadvantages described above.
U.S. Pat. No. 3,715,278 discloses the use of glass, in the form fibers or fabrics, as a support for enzymes, but does not disclose how to compensate for the relatively low surface area of glass.
U.S. Pat. No. 4,434,228 discloses the use of polymers of polyalkyleneimines and polycarboxylic acid which are mixed with enzymes to form particles. Post treatment with cross-linking agents is disclosed.
An immobilized enzyme composite and a method for immobilizing the enzyme on a bundle of non-porous filaments, such as glass, have now been discovered which provide an enzyme capable of retaining a high level of activity, an enzyme having long endurance and an enzyme which achieves a significantly high rate of conversion of enzyme substrate to the corresponding end product. Such an immobilized enzyme product will be particularly suited for commercial and biomedical processes utilizing enzyme catalysis, especially since the present invention overcomes the expense and inefficiencies of enzyme reactions conducted on a batch basis, as known in the past.